Molecular cloning of a G-protein alpha i subunit from the lobster olfactory organ

Immunolocalization of a Galphaq protein to the chemosensory organs of Dipolydora quadrilobata (polychaeta: spionidae)

Chemoreception in marine invertebrates mediates a variety of ecologically important behaviors including defense, reproduction, larval settlement and recruitment, and feeding. The sensory pathways that regulate deposit-feeding activity by polychaetes living in sedimentary habitats are of particular interest because such feeding has profound effects on the physical and chemical properties of the habitat. Nevertheless, little is known concerning the molecular mechanisms of chemical signal transduction associated with deposit feeding and other behaviors in polychaetes. Chemosensory-based feeding behaviors are typically regulated by G-protein-coupled signal transduction pathways. However, the presence and role of such pathways have not been demonstrated in marine polychaetes. Methodologies involving degenerate primer-based reverse transcription with the polymerase chain reaction and rapid amplification of cDNA ends were used to identify and characterize a Galphaq subunit expressed in the feeding palps of the spionid polychaete Dipolydora quadrilobata. The D. quadrilobata Galphaq protein had high sequence similarity with previously reported Galphaq subunits from both invertebrate and vertebrate taxa. Immunhistochemistry and immunocytochemistry were used with confocal laser scanning microscopy and transmission electron microscopy to visualize the distribution of a Galphaq antibody in whole worms and in cilia of the feeding palps. Galphaq immunoreactivity was concentrated in the nuchal organs, food-groove cilia, and lateral/abfrontal cilia of the feeding palps. Because these structures are known to be involved in chemoreception, we propose that Galphaq isolated from D. quadrilobata is a key component of chemosensory signal transduction pathways in this species.

Rosette-type tegumental glands associated with aesthetasc sensilla in the olfactory organ of the Caribbean spiny lobster, Panulirus argus

The lateral antennular flagellum of decapod crustaceans bears unique olfactory sensilla, namely the aesthetascs, and other sensilla types. In this study, we identify a new major tissue in the lateral flagellum of the Caribbean spiny lobster, Panulirus argus, namely "aesthetasc tegumental glands" (ATGs), based on immunostaining with antibodies against CUB serine protease (Csp), in situ hybridization with csp-specific probes, labeling with the F-actin marker phalloidin, labeling with the nuclear marker Hoechst 33258, and staining with methylene blue. Each ATG has 12-20 secretory cells arranged in a rosette. Each secretory cell has a Csp-immunoreactive basal portion and an apical portion containing granular material (metachromatic staining indicative of acid mucopolysaccharides). At the center of each secretory rosette is a phalloidin-positive common locus that gives rise to a main drainage duct projecting toward the cuticle. Scanning electron and light microscopy show that thin ducts traverse the cuticle and connect to "peg pores" proximal to the bases of the aesthetascs, with 3.4 peg pores per aesthetasc. Since the number of common loci is correlated with the number of peg pores, we conclude that each pore represents the outlet of one ATG, and that the secretions are released from them. We conclude further that ATGs and aesthetascs are functionally linked. We hypothesize that ATG secretions have antifouling and/or friction-reducing properties, and that they are spread over the surface of the aesthetascs by antennular grooming. A review of the literature suggests that ATGs are common in decapod crustacean antennules, and that rosette glands and grooming might be functionally coupled in other body areas.

Molecular cloning and characterization of crustacean type-one dopamine receptors: D1alphaPan and D1betaPan

Dopamine (DA) differentially modulates identified neurons in the crustacean stomatogastric nervous system (STNS). While the electrophysiological actions of DA have been well characterized, little is known about the dopaminergic transduction cascades operating in this system. As a first step toward illuminating the molecular underpinnings of dopaminergic signal transduction in the crustacean STNS, we have cloned and characterized two type-one DA receptors (DARs) from the spiny lobster (Panulirus interruptus): D(1alphaPan) and D(1betaPan). We found that the structure and function of these arthropod DARs are well conserved across species. Using a heterologous expression system, we determined that DA, but not serotonin, octopamine, tyramine or histamine activates these receptors. When stably expressed in HEK cells, the D(1alphaPan) receptor couples with Gs, and DA elicits an increase in [cAMP]. The D(1betaPan) receptor responds to DA with a net increase in [cAMP] that is mediated by Gs and Gz.

Olfactory specific chymotrypsin-like serine protease from the aesthetasc tegumental gland of the lobster, Homarus americanus

Numerous proteases and protease inhibitors are expressed in the lobster olfactory organ. One of these proteases, olfactory enriched transcript 03 (OET-03), is particularly interesting because its mRNA is expressed only in one cell type of the olfactory organ of the American lobster, Homarus americanus. We have obtained a full-length cDNA clone of OET-03. The predicted amino acid sequence is equally divided between a novel N-terminal domain and a conserved serine protease catalytic domain at the C-terminus. Heterologous expression in HEK293 cells allowed protease assays demonstrating that OET-03 cleaved a specific serine protease substrate, N-alpha benzoyl-L-arginine p-nitroanilide, but did not cleave a substrate of metalloproteases and cysteine proteases. OET-03 protease activity was significantly inhibited by the chymotrypsin-like protease inhibitor, tosyl-L-phenylalanine chloromethyl ketone, but not by the general protease inhibitor, phenylmethylsulfonyl fluoride. Immunoreactivity for OET-03 was detected only in the cells previously shown to contain OET-03 mRNA. The cytoplasm of these cells was filled with enlarged smooth endoplasmic reticulum (a characteristic of secretory cells) that appeared to expand into large electron-translucent areas at the ventral end of the cell. The ventral ends of these secretory cells were apposed to phalloidin-labeled triangular structures reminiscent of the beginnings of the ducts of crustacean tegumental glands. This putative gland was found only in association with the aesthetasc sensory units of the olfactory organ, hence the name, aesthetasc tegumental gland.

Inducible transcript expressed by reactive epithelial cells at sites of olfactory sensory neuron proliferation

The continuous replacement of cells in the spiny lobster olfactory organ depends on proliferation of new cells at a specific site, the proximal proliferation zone (PPZ). Using representational difference analysis of cDNA, we identified transcripts enriched in the PPZ compared to the mature zone (MZ) of the organ. The 12 clones identified included four novel sequences, three exoskeletal proteins, a serine protease, two protease inhibitors, a putative growth factor, and a sequence named PET-15 that has similarity to antimicrobial proteins of the crustin type. PET-15 mRNA was only detected in epithelial cells. It was abundant in all epithelial cells of the PPZ, but was only detected in the MZ at sites of damage to the olfactory organ. PET-15 mRNA was increased by types of damage that are known to induce proliferation of new olfactory sensory neurons in the olfactory organ. It increased in the PPZ after partial ablation of the olfactory organ and in the MZ after shaving of aesthetasc sensilla. These ipsilateral effects were mirrored by smaller increases in the undamaged contralateral olfactory organ. These contralateral effects are most parsimoniously explained by the action of a diffusible signal. Because epithelial cells are the source of proliferating progenitors in the olfactory organ, the same diffusible signal may stimulate increases in both cellular proliferation and PET-15 mRNA. The uniformity of expression of PET-15 in the PPZ epithelium suggests that the epithelial cells that give rise to new olfactory sensory neurons are a subset of cells that express PET-15.

Olfactory-enriched transcripts are cell-specific markers in the lobster olfactory organ

Genes expressed specifically in a tissue are often involved in the defining functions of that tissue. We used representational difference analysis of cDNA to amplify 20 cDNA fragments representing transcripts that were more abundant in the lobster olfactory organ than in brain, eye/eyestalk, dactyl, pereiopod, or second antenna. We then independently confirmed that the transcripts represented by these clones were enriched in the olfactory organ. The 20 cDNA fragments represent between 6 and 15 different genes. Six of the cDNAs contained sequences highly similar to known gene families. We performed in situ hybridization with these six and found that all were expressed in subsets of cells associated with the aesthetasc sensilla in the olfactory organ. Clones OET-07, an ionotropic receptor, and OET-10, an alpha tubulin, were specific to the olfactory receptor neurons. OET-02, a monooxygenase, was expressed only in the outer auxiliary cells. OET-03, a serine protease, was specific to the collar cells. OET-11, an alpha(2) macroglobulin, was expressed by the receptor neurons and the collar cells. OET-17, a calcyphosine, was expressed in the receptor neurons, inner auxiliary cells, and collar cells. The identities and expression patterns of these six transcripts predict involvement in both known and novel properties of the lobster olfactory organ.

Distribution of G-protein alpha subunits and neurotransmitter activation of g(alphai) and g(alphaq) in the brain of the lobster Homarus americanus

Immunocytochemistry using antisera specific for the G-protein alpha subunits G(alphai), G(alphaq), and G(alphas) revealed similar patterns of immunoreactivity in the lobster brain. Immunoreactivity was strongest in neuropil, especially the olfactory and accessory lobes, and was characterized by bundles of fine threads leading to dense concentrations of punctate staining in the glomeruli. This may reflect the concentration of G-protein alpha subunits at synapses. The major differences between the antisera were distinct patterns of staining intensity in subregions of glomeruli of the olfactory and accessory lobes. This result is potentially correlated with previous evidence that these subregions are neurochemically distinct. Neuronal cell bodies contained moderate levels of immunoreactivity at the plasma membrane and faint staining in the cytoplasm. The olfactory globular tract was moderately immunoreactive, but other fiber tracts were weakly immunoreactive. Immunoreactivity in the deutocerebral commissure consisted of small oval cell bodies and strands that formed a reticulated pattern, suggestive of glia. Photoaffinity labelling by using an analog of GTP demonstrated that histamine activated G(alphai) in brain homogenates. Further evidence of G-protein activation was obtained by showing that stimulation with a mixture of neuroactive substances increased the amount of phospholipase C-beta associated with membranes, G(alphaq), and G(beta). The lobster brain, especially in its neuropil regions, is richly endowed with neuromodulatory biochemical pathways involving G(alphai), G(alphaq), and G(alphas).

Lobster GABA receptor subunit expressed in neural tissues

A cDNA encoding an ionotropic gamma-aminobutyric acid (GABA) receptor subunit was isolated from a lobster (Homarus americanus) cDNA library. A longer version of this cDNA, containing a 108-bp insert, was also detected. The two cDNAs are predicted to encode alternatively spliced proteins of 485 and 521 amino acids, respectively. The sequences were most similar to the Drosophila RDL (resistance to dieldrin) GABA subunit with 54% identity, and 30-35% identity with vertebrate ionotropic GABA receptor subunits. Only the shorter clone formed functional ion channels when transfected into human embryonic kidney (HEK) 293 cells. GABA caused a Cl(-)-selective current in the presence of GABA that was blocked by picrotoxin. The GABA-induced current was weakly sensitive to the GABA(A) antagonist, bicuculline, but was enhanced by pentobarbital. Expression of the GABA receptor mRNA was highest in brain and the olfactory organ, but was not detected in leg muscle. These data suggest that the isolated cDNAs are likely to encode proteins that comprise subunits of native GABA receptors expressed in olfactory receptor neurons and projection neurons of the olfactory deutocerebrum.

Lobster G-protein coupled receptor kinase that associates with membranes and G(beta) in response to odorants and neurotransmitters

A cDNA clone (lobGRK2) encoding a protein of 690 amino acids with significant similarity to the GRK2 subfamily of G-protein coupled receptor kinases was isolated. lobGRK2 was widely expressed as a 9-kb major transcript and a protein of 80 kDa. It was most abundant in the brain and the olfactory organ but was absent in the eye/eyestalk. Immunocytochemistry revealed lobGRK2 immunoreactivity in the outer dendritic segments of the olfactory receptor neurons, the site of olfactory transduction. LobGRK2 immunoreactivity was observed in most neuronal structures in the brain, although with varying intensity. It was strongest in neuropil, especially the olfactory and accessory lobes but was also detectable in neuronal cell bodies. Stimulation of brain homogenates with a mixture of neurotransmitters increased the association of lobGRK2 with membranes and with G(beta). Similarly, stimulation of olfactory dendrite homogenates with an odorant mixture caused lobGRK2 to associate with G(beta). These results support the conclusion that lobGRK2 responds to odorants and to neurotransmitters and may act to initiate desensitization by phosphorylating G-protein-coupled receptors in the olfactory organ and the brain, respectively.

A lobster phospholipase C-beta that associates with G-proteins in response to odorants

A cDNA clone encoding a protein of 1116 amino acids with significant homology to beta-isoforms of phospholipase C was isolated from lobster olfactory organ cDNA libraries and named lobPLCbeta. This cDNA hybridized predominantly to a 9 kb transcript in RNA from olfactory organ, pereiopod, brain, and eye-eyestalk and to several smaller minor transcripts only in eye-eyestalk. An antiserum raised to the C terminus of lobPLCbeta detected immunoreactivity in a single 130 kDa band in olfactory aesthetasc hairs, olfactory organ, pereiopod, dactyl, and brain. In eye-eyestalk this 130 kDa band was abundant, and minor bands of 100, 79, and 57 kDa also were detected. In cross sections of the aesthetasc hairs, immunoreactivity was detected in the outer dendritic segments of the olfactory receptor neurons, the site of olfactory transduction. A complex odorant caused lobPLCbeta immunoreactivity to increase in membrane fractions and decrease in soluble fractions of homogenates of aesthetasc hairs. The odorant also increased the amount of lobPLCbeta in immunoprecipitates of Galphaq and Gbeta from homogenates of aesthetasc hairs. These results support the conclusion that lobPLCbeta mediates olfactory transduction.

Inhibition and enhancement of odorant-induced cAMP accumulation in rat olfactory cilia by antibodies directed against G alpha S/olf- and G alpha i-protein subunits

The odorant-induced accumulation of cAMP can be inhibited by antibodies directed against G alpha s/olf. In contrast, antibodies raised against G alpha i-subunits caused a strong enhancement of the odorant-induced cAMP accumulation. Western blotting and immunoelectron microscopy revealed the presence of both G alpha s/olf- and G alpha i-subunits in rat cilia preparations. The existence of both stimulatory and inhibitory odorant-induced regulation of adenylyl cyclase activity in olfactory cilia may indicate that an initial integration of different odorant stimuli begins at the level of primary reactions in the same effector enzyme.

Subclasses of vomeronasal receptor neurons: differential expression of G proteins (Gi alpha 2 and G(o alpha)) and segregated projections to the accessory olfactory bulb

Differential expression of G proteins (Gi alpha 2 and G(o alpha) and the separate central projections of Gi alpha 2- and G(o alpha)-immunoreactive (ir) vomeronasal receptor neurons were investigated in the mouse and rat using immunocytochemical methods. In the vomeronasal organ (VNO), receptor neurons with their cell bodies located in the middle layer (middle 1/3) of the vomeronasal sensory epithelium express Gi alpha 2. Axons of these Gi alpha 2-ir neurons can be followed from VNO to the anterior part, but not the posterior part, of the nerve-glomerular (N-GL) layer of the accessory olfactory bulb (AOB). Another population of receptor neurons, which are located in the deep layer (basal 1/3) of the vomeronasal sensory epithelium, express G(o alpha), and axons of the G(o alpha)-ir neurons can be traced to the posterior part, but not the anterior part, of the N-GL layers of the AOB. The axons of the two subclasses of receptor neurons are intermingled near the VNO and become segregated as they enter the AOB. Removal of the AOB results in retrograde degeneration of both Gi alpha 2-ir and G(o alpha)-ir receptor neurons in the VNO. These results suggest that at least two subclasses of receptor neurons exist in the VNO: the Gi alpha 2-ir neurons in the middle layer and the G(o alpha)-ir neurons in the deep layer of the VNO. The Gi alpha 2-ir neurons in the middle layer of the VNO project to the anterior part of the AOB, while the G(o alpha)-ir neurons in the deep layer of the VNO project to the posterior half of the AOB. These results are similar to our previous observations in the gray short-tailed opossum, suggesting that the existence of at least two subclasses of receptor neurons in the vomeronasal epithelium with differential projections to the AOB is a conserved feature among mammals.